This invention concerns a method of stabilizing polysaccharide biopolymer solutions employed in tertiary oil recovery at elevated temperature.
Tertiary oil recovery is commonly used today to recover the oil which remains in a subterranean petroleum-containing formation after primary and secondary recovery techniques have been exhausted.
One approach to tertiary oil recovery is polymer flooding. In this approach, certain organic polymers are added to at least a portion of the fluid injected into the formation to thicken the fluid and thereby improve the mobility of the entrapped oil as the fluid is driven from the injection site to the production well. Among these polymers are the xanthan gums, hydrophilic polysaccarides produced by fermentation using bacteria of the genus Xanthomonas. The biopolymer may be used in the form of the fermentation broth itself, as exemplified in U.S. Pat. No. 4,119,546, or in isolated and reconstituted form, as disclosed in U.S. Pat. No. 3,305,016. Xanthan gums are particularly desirable for polymer flooding since they are good displacing agents; give useful viscosities at low concentrations (5 to 90 centipoises at 100 to 3,000 ppm); are not lost by extensive adsorption of the porous rock formations; are relatively insensitive to salts, not precipitating or losing viscosity under normal conditions; and are reasonably shear stable and viscosity stable over a wide pH range. Other polysaccharide biopolymers include such as those derived from the genus Sclerotium and from the genus Alcaligenes, the use of which in enhanced oil recovery is described, for example, in U.S. Pat. No. 3,372,749 British Patent Specification No. 2,082,189, respectively.
While xanthan polysaccharide biopolymer solutions have proven to be effective in tertiary oil recovery, one problem which has arisen with their use is in oil reservoirs at elevated temperatures of about 60.degree.-150.degree. C. There, the polymer solution tends to be unstable with significant loss in viscosity in a few days, particularly in low-saline (500 ppm or less total dissolved solids) waters.
Solutions to this problem have been suggested. In U.S. Pat. No. 4,141,842, for example, the polymer solution includes a C.sub.3 to C.sub.5 aliphatic alcohol to stabilize the polysaccharide. In U.S. Pat. No. 4,218,327, the stability of the solution viscosity is improved by initially deoxygenating the aqueous liquid with agents such as sodium hydrosulphite (sodium dithionite) and then adding a sulfur-containing antioxidant, a readily oxidizable water-soluble alcohol or glycol, and the polymer. And in U.K. Patent Application G.B. No. 2000823A, aqueous solutions of polymeric mobility control agents, including polysaccharides, are stabilized with an alkylene polyamine, alkanolamine or alicyclic polyamine; the solutions may also contain an oxygen scavenger such as sodium hydrosulfite. PG,4
The need still exists for, and it is the primary objective of the present invention to provide, a simple, economical and effective means of stabilizing polysaccharide biopolymer flood control solutions employed at elevated temperature.